1. Field of the Invention
This invention relates to cubic boron nitride and its preparation. More particularly, the invention concerns reactive magnetron sputter deposition being employed to produce coatings comprising cubic boron nitride on substrates for providing abrasion resistant surfaces such as for cutting tools and the like useful products.
2. Description of Related Art
Magnetron sputtering apparati are described in patents such as commonly assigned U.S. Pat. No. 4,407,713 (Zega), incorporated herein by specific reference.
Production of cubic boron nitride (CBN) generally requires high pressure technology and processing techniques. Because of the complexity and inherent low volume of high pressure processing, the presently produced cutting tools with CBN coatings are expensive.
Due to its hardness, abrasion resistance, thermal and mechanical shock resistance, CBN would be wider used if economical processes for its production were available and the geometry of the parts was not so restricted.
It is known that hexagonal boron nitride (BN) can be converted to the cubic form through treatment at high temperatures (1800.degree. C.) and pressures (85 kilobars) with this conversion catalyzed by various materials, such as alkali and alkaline earth metals. A synthesis of CBN from BN is described in U.S. Pat. No. 3,918,219 (Wentorf).
U.S. Pat. No. 3,791,852 (Bunshah) describes a process and apparatus for the production of carbide films by physical vapor deposition in what can be termed to be an activated reactive evaporation deposition technique.
U.S. Pat. Nos. 4,297,387, 4,412,899 and 4,415,420 (Beale) describe activated reactive evaporation methods for depositing CBN on a substrate. These methods seek to form CBN in situ in an ionizing electrical field from reactive constituents comprising metal vapors such as typically boron and aluminum together with ammonia gas or nitrogen to enable nitride formation.
Reactive evaporation methods tend to incorporate contaminants such as oxygen into the deposited coatings. Oxygen incorporation gives rise to formation of B.sub.2 O.sub.3 brittle coatings. Reactive evaporation methods give results inconsistent in reproducibility. It is not known if the inconsistent reproducibility is attributable to critical configurations in the methodology or whether attributable to the recently discovered short life span of CBN with reversion to amorphous forms upon standing.
Chemical vapor deposition methods require high substrate temperatures which preclude coating tool steel, plastic, or integrated circuits. Ion beam deposition, ion plating and electron beam evaporation are lower temperature processes. However, ion beam deposition suffers from low coating rate (.about.100 .ANG./min) and short operating life of the coating apparatus while ion plating and electron beam evaporation suffer from problems associated with process control which makes commercialization difficult and expensive.
Surprisingly, CBN appears to be unstable over time reverting from a cubic to an amorphous form within as little as 3 months. Exclusion of oxygen or water does not seem to retard the reversion. The short lived stability of CBN creates a need for low cost, high quality deposition methods which can be useable on site to take advantage of the full life of the CBN.